The paint cans most widely used at the present time employ a cylindrical metal container which is provided with a double-tight metal sealing ring attached to a top rim of the container. The primary advantage of this old design is that the double-tight sealing ring affords an excellent seal which can be opened and resealed repeatedly.
Notwithstanding the popularity of the metal paint cans with double-tight metal sealing rings and their extensive use in the paint industry, these cans do suffer from some rather significant disadvantages. For instance, the conventional metal can is expensive to produce because of the costs involved in purchasing the sheet metal from which the can is fabricated and then coating the can to prevent rust. A rust preventer is also applied to the sheet metal used to make the double-tight sealing ring. However, when the sheet metal is cut to form a ring blank from which the sealing ring is formed, subsequently a raw uncoated inner end of metal remains after the sealing ring has been formed. Thus, another problem is that paint may be discolored by rust which forms on the inner end of the sealing ring and falls into the container. The sealing ring fabrication process is also time consuming and expensive.
In an effort to avoid the problems and disadvantages discussed above, plastic paint cans have been proposed. Many of these plastic paint cans employ a snap-on lid which, because of the resiliency required to snap onto a rim of the can, must also be made from plastic or a similar material (see, for instance, U.S. Pat. Nos. 3,609,263; 3,804,289; 4,037,748 and 4,210,258). One problem associated with such plastic paint cans is that the plastic lids can be punctured or ruptured more easily than their metal counterparts. Another disadvantage is that the prior art paint cans which employ snap-on lids are not compatible with the plug-type lids employed by conventional metal paint cans. Also, many of these plastic paint cans are not compatible with existing paint can handling, filling and bailing equipment.
Plastic paint cans have also been proposed which employ plug-type lids (see, for instance, U.S. Pat. Nos. 3,730,382; 3,817,420 and 4,201,311). These lids are adapted to engage sealing rings or similar elements formed monolithically with the cans. A major problem associated with these plastic paint cans is that they lack the necessary rigidity to maintain their cylindrical shape, and, therefore, they tend to distort when subjected to a large load, such as when the cans are stacked one on top of the other.
So as to avoid the problems associated with the plastic paint cans described above, the assignee of the present invention and application developed a unique plastic paint container having a double-tight metal sealing ring which, due to its metallic construction, enhances the rigidity of the container (see U.S. Pat. No. 4,228,916). Moreover, because a conventional double-tight metal sealing ring is employed, the container can be sized and shaped so as to be compatible with existing paint can filling, handling and bailing equipment. Although the assignee of the present invention and application has successfully marketed this composite plastic paint container for a number of years, there is still room for additional improvement. For instance, the metal sealing ring is relatively expensive. Also, because of its metallic construction, the sealing ring increases the overall weight of the container and still poses a potential rust problem. A still further disadvantage is that the metal sealing ring adds an additional manufacturing step in order to seam the sealing ring to the container. If the metal sealing ring of the paint container disclosed in U.S. Pat. No. 4,228,916 were replaced by a plastic sealing ring, the seaming operation could be dispensed with and the sealing ring would be attached to the container by a suitable adhesive. Alternatively, the seaming operation used to attach the metal sealing ring to the container could be replaced by an induction welding operation, such as the one disclosed in U.S. Pat. No. 2,741,402 which involves inserting a metal ring in a seam between two container parts and subjecting the assembly to induction heating, the metal ring localizing the heat in the vicinity of the seam to facilitate the induction welding operation.
An essentially all plastic paint container made from a plastic bucket and a separate plastic sealing ring has not been developed in the past. One reason is that the use of a separate plastic sealing ring would still require an additional manufacturing step (i.e., an induction welding operation), as compared with the much more popular technique of forming the sealing ring monolithically with the container. Thus, the concept of forming a plastic paint container from a plastic bucket and a separate sealing ring, whether plastic or metallic, represents a significant departure from the conventional and widely accepted practice of forming plastic paint containers monolithically (i.e., by molding the plastic sealing ring and the container as a unitary structure). Another reason for the lack heretofore of a plastic paint container made from a plastic bucket and a separate plastic sealing ring is that while the use of a metal sealing ring has the advantage of rigidifying the plastic container, the use of an unreinforced plastic sealing ring would not provide such an advantage.